Human activity is causing irreversible harm to the environment. The level of carbon dioxide and other greenhouse gases in the atmosphere exceeds preindustrial levels by nearly 40 percent and is rising rapidly. This blanket of heat-trapping gases is already largely responsible for increasing the earth's average surface temperature by 0.7 degrees Celsius. If current fossil-fuel-consumption trends continue, average surface temperatures could rise by as much as 6.4 degrees by 2100, according to the Intergovernmental Panel on Climate Change. Even under the IPCC's most optimistic scenario, temperatures will still rise by 1.1-2.9 degrees before century's end. An increase of more than two degrees could have serious adverse impacts, including the extinction of many plant and animal species or even the collapse of entire ecosystems.

The economic costs of unchecked global warming will be severe. Precise quantification is difficult given the myriad uncertainties and subjective judgments involved in making such calculations. In 2007, the IPCC estimated that global warming could lead to continuing global GDP losses of one to five percent and even greater losses at the regional and local levels. Climate change is also beginning to create major security risks. The Age of Consequences, a report released in 2007 by the Center for Strategic and International Studies, noted that if the planet warms by 1.3 degrees by 2040 (which is what current projections indicate will occur), there will be "heightened internal and cross-border tensions caused by large-scale migrations; conflict sparked by resource scarcity . . . ; increased disease proliferation . . . ; and some geopolitical reordering."

Containing climate change will require reducing the current levels of greenhouse gas emissions not only in the United States and other wealthy countries but also in rapidly developing nations such as China. Per capita emissions in the United States today are four times as great as those in China and 20 times as great as those in India. But China has already overtaken the United States as the world's largest overall emitter of carbon dioxide. Even if the wealthy countries cut their total greenhouse gas emissions by 80 percent by midcentury, aggregate